1. Field of the Invention
The present invention relates to an absolute rotary encoder.
2. Description of the Related Art
An incremental rotary encoder or absolute rotary encoder is conventionally used to measure an angle. The incremental rotary encoder radially records slits having a predetermined period on a disk, optically or magnetically reads the relative rotational movement of the slits, and computes an absolute rotation angle by combining the read result with an origin detecting mechanism. Note that many high-resolution products each of which has a slit pitch decreased to about 80 μm and interpolates a phase within one pitch by dividing the phase into about 10,000 portions by using an electrical divider are recently available. Also, the slits are normally not read one by one, but a plurality of slits are read by optically averaging them. This increases the accuracy because dust, flaws, defects, and pattern variation errors of the slits themselves are averaged. Since, however, absolute angle information cannot be obtained unless the original is initially detected, applications to particularly the fields of machine tools and robots are limited.
Various kinds of absolute encoders have been proposed as binary patterns, and a so-called gray code method of recording binary patterns on a plurality of tracks and a method of recording a random-number code on one track are available. However, the resolution of the gray code method is not so high because mounting errors make it difficult to synchronize the detection timings of pieces of information of different tracks. Accordingly, a method has been proposed by which binary codes 1 and 0 are expressed by the value of the transmittance, the value of the slit width, or the presence/absence of slits or pits arranged at equal intervals, and recorded as cyclic code patterns on one track. The cyclic code patterns are patterns in which when 1s and 0s are arranged at random, there are absolutely no portions having the same arrangement on the entire circumference in M adjacent patterns. An absolute rotary encoder described in Japanese Patent Laid-Open No. 60-89713 adopts a method using a scale board having a scale in which scale marks are repetitively formed at a predetermined pitch and the length in the repetitive direction sequentially changes by a predetermined amount.
Japanese Patent Laid-Open No. 2004-529344 has disclosed an absolute encoder using an absolute code obtained by partially removing reflective slits periodically arranged at equal intervals.
Unfortunately, unique problems arise when applying the one track type absolute encoder as described above to a rotary encoder having a scale formed on a cylindrical surface (a surface of a circular cylinder). The first problem is that when illuminating marks formed on the cylindrical surface with a light beam and detecting light reflected by the marks by a light receiving element array, the projection magnification fluctuates because a distance d between the cylindrical surface and a light projecting/receiving unit (head) changes. To solve the first problem, Japanese Patent Laid-Open No. 2008-539409 has disclosed a method of installing a sensor for detecting the value of the distance, thereby arranging the cylindrical surface and head at an appropriate distance d.
The second problem is periodic distortion. The periodic distortion is a phenomenon in which especially when a radius R of the cylindrical surface is small, the enlargement magnification increases when a light beam illuminating the peripheral portion of the cylindrical surface is projected onto a linear light receiving element array, compared to a light beam illuminating the front portion of the cylindrical surface. If this periodic distortion exists, the period of bright-and-dark patterns on the light receiving element array prolongs toward the peripheral portion. Japanese Patent Laid-Open No. 2008-539409 gives no suggestion as to this periodic distortion.
In the one track type absolute rotary encoder, a code error or phase detection (interpolation) error occurs unless the intensity and phase information of a projected pattern are accurately detected in both the central portion and peripheral portion of the light receiving element array. Conventionally, therefore, only a product in which a scale is formed on a cylindrical surface that has a relatively large R (about 50 mm) and hence is approximately regarded as a flat surface has been put to practical use.
Accordingly, an absolute rotary encoder having a scale on a cylindrical surface is required to be capable of canceling the influence of the periodic distortion, and the change in projection magnification of marks caused by the change in distance between the cylindrical surface and a head, and also capable of downsizing the cylindrical surface. In addition, to put a so-called module type encoder, in which a cylindrical surface and head are separated, to practical use, it is important to allow some tolerance to a mounting error, and include a function of outputting information indicating the mounting state is proper. Accordingly, a demand has arisen for an absolute rotary encoder including these functions.